We have measured basal levels of free intracellular calcium in platelets before and after standardized stimulation with ADP. The ADP-induced increase in intracellular calcium is blocked by prostaglandins I2, D2, E1, and E2, but not by F2 alpha. The increase in cytosolic calcium is also blocked by dbcAMP, a synthetic analog of cAMP, which is known to be increased by prosta-glandins and forskolin, which blocks the metabolism of cAMP. The increase is partially dependent on extracellular calcium, but cannot be blocked by calcium channel antagonists. We have developed a method to grow vascular smooth muscle cells (VSMC) in monolayers on cover slips and to measure the intracellular levels of free calcium with a fluorescent agent, Fura2, in this state. Preliminary studies have shown that certain types of receptors are lost from these cells during serial passages.